The present invention relates generally to antenna apparatuses and systems, and more particularly, to planar antennas with non-dispersive, ultra wide bandwidth (UWB) characteristics.
With respect to the antenna of radar and communications systems, there are five principle characteristics relative to the size of the antenna: the radiated pattern in space versus frequency, the efficiency versus frequency, the input impedance versus frequency, and the dispersion. Typically, antennas operate with only a few percent bandwidth, and bandwidth is defined to be a contiguous band of frequencies in which the VSWR (voltage standing wave ratio) is below 2:1. In contrast, ultra wide bandwidth (UWB) antennas provide significantly greater bandwidth than the few percent found in conventional antennas, and exhibit low dispersion. For example, as discussed in Lee (U.S. Pat. No. 5,428,364) and McCorkle (U.S. Pat. Nos. 5,880,699, 5,606,331, and 5,523,767), UWB antennas cover at least 5 or more octaves of bandwidth. A discussion of other UWB antennas is found in xe2x80x9cUltra-Wideband Short-Pulse Electromagnetics,xe2x80x9d (ed. H. Bertoni, L. Carin, and L. Felsen), Plenum Press New York, 1993 (ISBN 0-306-44530-1).
As recognized by the present inventor, none of the above UWB antennas, however, provide high performance, non-dispersive characteristics in a cost-effective manner. That is, these antennas are expensive to manufacture and mass-produce. The present inventor also has recognized that such conventional antennas are not electrically small, and are not easily arrayed in both 1D (dimension) and 2D configurations on a single planar substrate. Additionally, these conventional antennas do not permit integration of radio transmitting and/or receiving circuitry (e.g., switches, amplifiers, mixers, etc.), thereby causing losses and system ringing (as further described below).
Ultra wide bandwidth is a term of art applied to systems that occupy a bandwidth that is approximately equal to their center frequency (e.g., greater than 50% at the xe2x88x9210 dB points). A non-dispersive antenna (or general circuit) has a transfer function such that the derivative of phase with respect to frequency is a constant (i.e., it does not change versus frequency). In practice, this means that an impulse remains an impulsive waveform, in contrast to a waveform that is spread in time because the phase of its Fourier components are allowed to be arbitrary (even though the power spectrum is maintained). Such antennas are useful in all radio frequency (RF) systems. Non-dispersive antennas have particular application in radio and radar systems that require high spatial resolution, and more particularly to those that cannot afford the costs associated with adding inverse filtering components to mitigate non-linear antenna phase distortion.
Another common problem as presently recognized by the inventor, is that most UWB antennas require balanced (i.e., differential) sources and loads, entailing additional manufacturing cost to overcome. For example, the symmetry of the radiation pattern (e.g., azimuthal symmetry on a horizontally polarized dipole antenna) associated with balanced antennas can be poor because of feed imbalances arising from imperfect baluns. Furthermore, the balun, instead of the antenna, can limit the antenna system bandwidth due to the limited response of ferrite materials used in the balun. Traditionally, inductive baluns are both expensive, and bandwidth limiting. Furthermore, other approaches used to deal with balanced antennas utilize active circuitry to build balanced (or differential) transmit/receive (TR) switches, differential transmitters, and differential receivers, in an effort to maximize the bandwidth at the highest possible frequencies. Such approaches, however, are more costly than simply starting with unbalanced antenna constructions.
Another problem with traditional UWB antennas is that it is difficult to control system ringing. Ringing is caused by energy flowing and bouncing back and forth in the transmission line that connects the antenna to the transmitter or receiverxe2x80x94like an echo. From a practical standpoint, this ringing problem is always present because the antenna impedance, and the transceiver impedance are never perfectly matched with the transmission line impedance. As a result, energy traveling either direction on the transmission line is partially reflected at the ends of the transmission line. The resulting back-and-forth echoes thereby degrade the performance of UWB systems. In other words, is, a clean pulse of received energy that would otherwise be clearly received can become distorted as the signal is buried in a myriad of echoes. Ringing is particularly problematic in time domain duplex communication systems and in radar systems because echoes from the high power transmitter obliterate the microwatt signals that must be received nearly immediately after the transmitter finishes sending a burst of energy. The duration of the ringing is proportional to the product of the length of the transmission line, the reflection coefficient at the antenna, and the reflection coefficient at the transceiver.
In addition to distortion caused by ringing, transmission lines attenuate higher frequencies more than lower frequencies, and sometimes delay higher frequency components more than lower frequency components (i.e. dispersion). Both of these phenomena cause distortion of the pulses flowing through the transmission line. Thus it is clear that techniques that allow shortening of the transmission line have many advantagesxe2x80x94reducing loss, ringing, gain-tilt, and dispersion.
In view of the foregoing, there exists a need in the art for a simple UWB antenna that has an unbalanced feed, and can be arrayed in 1D and 2D on a single substrate (i.e., planar or conformal). Additionally, there is a need for a UWB antenna that is electrically small yet has low VSWR and allows the transmit and or receiving circuits to be integrated onto the same substrate to eliminate transmission line losses, dispersion, and ringing. Furthermore, there is a need for a UWB that can be mass-produced inexpensively.
Accordingly, an object of this invention is to provide a novel apparatus and system for providing an electrically small planar UWB antenna.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that is inexpensive to mass-produce.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that has a direct unbalanced feed that can interface to low-cost electronic circuits.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that has a flat frequency response and flat phase response over ultra wide bandwidths.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that exhibits a symmetric radiation pattern.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that is efficient, yet electrically small.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that integrates with the transmitter and receiver circuits on the same substrate.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that is planer and conformal, so as to be capable of being easily attached to many objects.
It is also an object of this invention to provide a novel apparatus and system for providing a UWB antenna that does not require an active electronic means or passive means of generating and receiving balanced signals.
It is a further object of this invention to provide a novel apparatus and system for providing a UWB antenna that can be arrayed in both 1D and 2D, in which the array of UWB antennas are built on single substrate with the radiation directed in a broadside pattern perpendicular to the plane of the substrate.
These and other objects of the invention are accomplished by providing a tapered clearance area (or clearance slot) within a sheet of conductive material, where the feed is across the clearance area. A ground element, which can be made of a conductive material such copper, has a xe2x80x9cholexe2x80x9d cut in it that is defined by the outer edge of the clearance area. A driven element, which is situated in the clearance area, is defined by the inner edge of the clearance area. The clearance area width at any particular point, measured as the length of the shortest line connecting the ground and the driven element, roughly determines the instantaneous impedance at that point. In some embodiments of the present invention, the clearance area width is tapered to increase as a function of the distance from the feed point, so that the impedance seen at the feed, for example with a time domain reflectometer (TDR), is tapered smoothly in the time domain.
Also in some embodiments of the present invention, the clearance area width, as well as the shape of the driven element, has an axis of symmetry about the line cutting through the feed point and the point on the driven element opposite the feed point. For example, the driven element can be circular, and the ground xe2x80x9cholexe2x80x9d can be a larger circle, wherein the centers are offset, such that the slot-width grows symmetrically about its minimum. The feed point is at the minimum width, in which the maximum width is on the opposite side, thus forming an axis of symmetry about the feed.
According to some embodiments of the present invention, the feed is at the minimum width. According to some embodiments, the ground xe2x80x9cholexe2x80x9d is oval shaped, and the driven element is oval with a depression in the side opposite the feed element. According to other embodiments, the ground xe2x80x9cholexe2x80x9d is oval shaped with a bulge in the side opposite the feed element, and the driven element is oval. According to still other embodiments, the ground xe2x80x9cholexe2x80x9d is oval shaped with a bulge in the side opposite the feed element, and the driven element is oval with a depression in the side opposite the feed element. An important factor is that the input impedance is tapered in the time domain in such a way as to provide the desired performance.
The antenna can be fed by connecting a coaxial transmission line to the feed point such that the shield of the coaxial cable is connected to the ground at the edge of the clearance area, and the center conductor of the coaxial cable is connected to the driven element also at the edge of the clearance area.
In some embodiments the ground element is cut to occupy only a thin perimeter so that the entire antenna is electrically small.
In order to meet these and other objects of the invention, an antenna device is provided having ultra wide bandwidth (UWB) characteristics. The antenna device includes a ground element having a cutout section with an inner circumference, the inner circumference having a first shape; and a driven element with an outer circumference having a second shape, the driven element being smaller in size than the cutout section and being situated within the cutout section to define a clearance area between the driven element and the ground element. The first shape may be a first simple closed curve having no cusps. The second shape may be a second simple closed curve having no cusps, including at least a concave portion and a convex portion. The first and second shapes may be formed such that any radial line from the center point of the driven element will intersect the first shape at a single first intersection point, and will intersect the second shape at a single second intersection point, a distance on the radial line between the first and second intersection points being defined as a clearance width between the driven element and the ground element for the radial line. The clearance area may be tapered such that a clearance width between the driven element and the ground element is monotonically nondecreasing from a minimum clearance width to a maximum clearance width.
The antenna device may further include a transmission line for providing an electrical signal to the driven element. The transmission line may be connected to a driven element at a feed point proximate to the minimum clearance width of the clearance area. The transmission line comprises a metal layer, a magnet wire, a coaxial cable, or other connection device. The transmission line may non-coplanar with either the driven element or the ground element.
The clearance area may be filled with one of FR-4, Teflon, fiberglass, or air. The ground element and the driven element may comprise a conductive material, and that conductive material may be copper.
The first and second shapes may be the same, except in different scale. The concave portion of the second shape may be formed proximate to the maximum clearance width. The driven element may have an axis of symmetry about a line that passes between the minimum clearance width of the clearance area and the maximum clearance width of the clearance area. The concave portion of the second shape may be centered on the axis of symmetry, proximate to the maximum clearance width.
An antenna device having ultra wide bandwidth (UWB) characteristics is also provided, including a ground element having a cutout section with an inner circumference, the inner circumference having a first shape; and a driven element with an outer circumference having a second shape, the driven element being smaller in size than the cutout section and being situated within the cutout section to define a clearance area between the driven element and the ground element. The first shape may be a first simple closed curve having no cusps, including at least a concave portion and a convex portion. The second shape may be a second simple closed curve having no cusps, including at least a concave portion and a convex portion. The first and second shapes may be formed such that any radial line from the center point of the driven element will intersect the first shape at a single first intersection point, and will intersect the second shape at a single second intersection point, a distance on the radial line between the first and second intersection points being defined as a clearance width between the driven element and the ground element for the radial line. The clearance area may be tapered such that a clearance width between the driven element and the ground element is monotonically nondecreasing from a minimum clearance width to a maximum clearance width.
The antenna device may further include a transmission line for providing an electrical signal to the driven element. The transmission line may be connected to a driven element at a feed point proximate to the minimum clearance width of the clearance area. The transmission line comprises a metal layer, a magnet wire, a coaxial cable, or other connection device. The transmission line may non-coplanar with either the driven element or the ground element.
The clearance area may be filled with one of FR-4, Teflon, fiberglass, or air. The ground element and the driven element may comprise a conductive material, and that conductive material may be copper.
The first and second shapes may be the same, except in different scale. The concave portion of the second shape may be formed proximate to the maximum clearance width. The driven element may have an axis of symmetry about a line that passes between the minimum clearance width of the clearance area and the maximum clearance width of the clearance area. The concave portion of the second shape may be centered on the axis of symmetry, proximate to the maximum clearance width.
With these and other objects, advantages and features of the invention that may become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings herein.